1. Field of the Invention
The present invention relates to printing methods, printing apparatuses, printing systems, and correction patterns for inhibiting darkness non-uniformities in an image.
2. Description of the Related Art
Inkjet printers that form dots by ejecting ink onto paper serving as a medium are known as printing apparatuses for printing images. Such inkjet printers repeat a dot formation operation of forming dots on the paper by ejecting ink from a plurality of nozzles, which move in a predetermined movement direction, and a carrying operation of carrying the paper in an intersecting direction that intersects with the movement direction (hereinafter, referred to as the “carrying direction”) with a carry unit. Thus, a plurality of raster lines each made of a plurality of dots extending in the movement direction are formed in the carrying direction, thus printing an image.
Now, with such a printer, darkness non-uniformities extending parallel to the movement direction can be occasionally observed in such images made of a multitude of raster lines. That is to say, there may be raster lines that macroscopically look darker and raster lines that macroscopically look lighter.
Such darkness non-uniformities are mainly caused by the manufacturing precision of the nozzles, and more specifically the following two causes can be given. A first case is that there are variations in the ink ejection amount among the nozzles. A second case is that the positions at which dots are formed on paper by ejecting ink from the nozzles (referred to as “dot formation positions” in the following) deviate in the carrying direction from the target positions.
The following is a conventional method for inhibiting darkness non-uniformities in the first case: First, a correction pattern is printed with the printer for which darkness non-uniformities are to be inhibited, the darkness of the correction pattern is measured with a darkness measurement device and a correction value with which a target darkness is attained is calculated for each nozzle, and the correction values for the nozzles are stored in a storage section of the printer. Then, when the user carries out actual printing of an image, the printer corrects the ink ejection amount using the correction values correlated with the nozzles, thus inhibiting the darkness non-uniformities.
However, the darkness non-uniformities of the second case are caused by the spacings between the raster lines R periodically becoming wider and narrower, as shown in FIG. 25. That is to say, adjacent raster lines R with a wide spacing between them macroscopically appear light, whereas raster lines R with a narrow spacing between them macroscopically appear dark. Thus, the state of the spacing depends on the combination of nozzles forming the adjacent raster lines. Consequently, in order to inhibit the darkness non-uniformities of the second case, it would appear to be possible to correct the darkness non-uniformities caused by the state of the spacings more directly by storing a correction value in correlation with each of the raster lines rather than storing a correction value in correlation with each of the nozzles in the storage section.
However, the number of raster lines constituting the image is extremely large, and a storage section of an enormous capacity would be necessary in order to store correction values for all of the raster lines, which would entail higher costs for the printer.
Moreover, as a method for obtaining the correction values, it is conceivable to form a large number of raster lines over the entire paper in the carrying direction as a correction pattern, and to determine the correction values by measuring the darkness thereof raster line by raster line. However, depending on the processing mode, which determines the dot formation operation and the carrying operation, there are also cases in which it is possible to obtain the correction values that are minimally necessary for correction by forming raster lines not over the entire region but only a portion thereof. In such a case, it is possible to achieve savings in the ink amount that is consumed during printing of the correction pattern.